Implantable medical devices typically rely on implanted power sources, such as a battery. Such batteries are commonly not rechargeable, and their life expectancy may be less than the life expectancy of the patient in which the device is implanted. Therefore the device may need to be explanted from the patient in order to change the power source. Determining the correct time to replace the battery is important for a number of reasons. Premature replacement can add unnecessary stress and trauma to the patient due to the surgery required to explant the device. However, delaying replacement of the battery could mean that the power source depletes to a level where the device cannot function as intended.
Cardiac pacemakers and implantable cardioverter-defibrillator (ICD) devices are two types of implantable medical devices that must function properly and reliably for patient survival, and there has been significant research and development in monitoring battery life for these types of devices. One of the challenges with monitoring battery life for pacemakers and ICDs is that these devices deliver variable therapies, often having an on-demand or as-needed therapy schedule. Because the therapies delivered by these devices are variable and not consistent over time, power consumption is unpredictable.
A number of methods have been used in the past to determine when to replace the battery in an implantable device such as a pacemaker or ICD. A first method of determining an elective replacement period is based on a “worst-case scenario” schedule. While such a replacement period is simple to calculate, the device is often replaced very prematurely. The operating parameters of pacing devices and cardioverter-defibrillators may vary widely over the life of the device either because of physiological changes in the patient, or because of marked changes in the patient's activity or condition. Therefore, the power consumed by these devices can vary over the lifetime of the device. As a result, an undesirably wide margin of error in the battery life prediction (the worst-case scenario) must be used to guard against these eventualities, thereby forcing premature surgical replacement of the implanted device and its battery in many cases, with the attendant risks of complications to the patient.
Monitoring one or more parameters of the battery, such as voltage or impedance, is another method to determine the elective replacement period of an implantable device, although this method has one significant shortcoming. For batteries having certain battery chemistries such as lithium-based batteries, the voltage of the battery will commonly very slowly decline over time with only a slight variation in voltage until the battery nears the end of its useful life. As the battery nears the end of its useful life, the battery voltage will begin to decline at a greater rate, often dramatically with a sharp drop-off in voltage. Such a battery is advantageous as a source of power for an implantable device because the battery delivers such an assured relatively constant voltage over most of the useful life of the device. However, such a battery creates a problem for a battery longevity monitor using the voltage of the battery to determine the longevity of the battery. Since the battery voltage remains relatively constant over most of the life of the battery, it is difficult to predict whether the battery is in the early part of the relatively flat voltage curve or nearing the end of the relatively flat voltage curve. The difference, of course, can mean a dramatic difference in the predicted longevity of the battery.
A further drawback to monitoring battery voltage and/or impedance to determine battery life is that the tolerances in battery voltage measurements as well as battery impedance may have such a wide variance in tolerances so as to render meaningless any estimation for battery replacement based on measurements of small changes in these values. Many implantable devices are thus prematurely explanted based upon these inadequate measurements.
Another method of determining replacement time of an implantable device such as a pacemaker is by measuring the energy consumed by the device. Since the battery energy at the time of implantation is known, circuitry can be included to subtract the energy consumed from the initial amount of energy to provide an approximate end of life for the battery. The basic concept of detecting end-of-life (EOL) by accumulating a measure of energy usage in an implanted pacemaker is illustrated in U.S. Pat. Nos. 4,556,061 and 4,715,381.
In U.S. Pat. No. 4,556,061, the invention relies on circuitry having an extremely precise capacitor to obtain an end of life for the battery. The device includes a counter which continuously accumulates the emitted pulses so as to provide a measure of the integral of battery current flow, and thus total energy expenditure. Unfortunately, the expected variability of the capacitor value over the lifetime of an implanted pacemaker, which may be ten or more years, leads to a loss of accuracy in the prediction of an appropriate time for battery replacement.
U.S. Pat. No. 4,715,381 illustrates a technique of making calculations of approximate battery energy expenditure, rather than actually measuring battery consumption. This reference shows a stimulation pulse counter which counts the number of delivered stimulus pulses. This information is utilized together with the programming parameters to determine the total amount of energy of the delivered pulses over an elapsed time. This calculated signal is added to a fundamental consumption signal which is based upon certain approximations and assumptions, and used to derive a signal representative of approximate total battery expenditure. This technique clearly provides at best an approximation, and is inherently subject to a greater probability of inaccuracy than the energy consumption technique. Further, this invention requires entering a “test mode” to determine end of battery life.
Another method of determining a battery replacement date is disclosed in U.S. Pat. No. 6,901,293, whereby a battery voltage monitor is combined with an energy counter. The device uses the data received from both a battery voltage monitor and an energy counter to determine an estimated battery replacement date.
A need still exists for an improved way of determining a battery replacement or recharging date for implantable medical devices such as therapy devices and monitoring devices.